Portable communication devices, WLAN routers etc. or more generally: transmission/reception devices which communicate using RF signals, require RF filters in order to separate desired signals from undesired signals. Such filters can be connected, for example, in front-end circuits, for example in duplexers.
In this case, the filter should look after the task of distributing the signals between a chipset and filters which are possibly also present. The circuit complexity should be as low as possible. The filter should be compatible with a multiplicity of different filter technologies of further filters, should enable a small size of a corresponding component and, in particular, should allow high selectivity.
The trend for communication devices which can serve more and more frequency bands results in complex connections of different filters for the different frequency bands. Therefore, the need arises for tunable filters in order to be able to use different frequency bands with the same filter.
Previous solutions for these requirements are based substantially on expanding known filter circuits with tunable impedance elements or on the use of switches which can be used to connect filter elements to form a filter topology.
The article “Tunable Filters Using Wideband Elastic Resonators”, Kadota et al., IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, Vol. 60, No. 10, October 2013, pages 2129-2136, discloses filter circuits in which tunable capacitors are added to RF filters having acoustic resonators.
The article “A Novel Tunable Filter Enabling Both Center Frequency and Bandwidth Tunability”, Inoue et al., Proceedings Of The 42nd European Microwave Conference, Oct. 29-Nov. 1, 2012, Amsterdam, The Netherlands, pages 269-272, discloses RF filters having tunable capacitors and tunable inductances.
The article “RFMEMS-Based Tunable Filters”, Brank et al., 2001, John Wiley & Sons, Inc. Int J RF and Microwave CAE11: pages 276-284, 2001, also discloses connections of L and C elements, the capacitances of the capacitive elements being adjustable.
The article “Tunable Isolator Using Variable Capacitor for Multi-band System”, Wada et al., 978-1-4673-2141-9/13/$31.00, 2013 IEEE MTT-S Symposium and the published document WO2012/020613 disclose the use of isolators in RF filters.
In the LTE-A (Long-Term Evolution-Advanced) mobile radio standard, mobile telephones can be operated in the so-called carrier aggregation mode (CA) in order to enable high data rates during downlink and/or uplink operation. In the CA mode, transmission and reception operation is possible in a first frequency band, while reception can be carried out at the same time in a further frequency band. By convention, different band combinations were proposed for this purpose, preferably inter-band combinations in which the frequency bands differ considerably in the CA mode. The greater the frequency difference between the frequency bands to be combined in the CA mode, the easier the technical implementation.
Band combinations in which the frequency bands to be combined are in the same frequency range or in closely adjacent frequency bands have additionally been proposed. Promising technical implementations which can be implemented using a single antenna have not yet been proposed for such in-band CA modes.